To start farming you are going to want to go to Catherby. You can get to Catherby via teleport in any spellbook.
Normal
Lunar
Ancients
Once you're there follow this.
Buy compost, watering can, a rake, seed dibber, spade, and secateurs from the farming shop. Buy some potato seeds aswell, you will need 3 per patch you're planting them in. Now go back out of the farming shop and use the rake on one of the two allotments.
Use the rake on it 3 times until there is no more weeds.
With the weeds you get from raking you can drop them, they are useless.
Now use the compost on the allotment
Compost makes it so your allotment or any plant won't get diseased. If your plant does get diseased then you can buy a plant cure from the Farming Shop.
Plant the seeds after you used the compost.
Then use the watering can on it.
Wait 30 minutes for it to grow. You can set a timer on Countdown Timer
Your seed will go through 5 stages while growing.
Phase 1
Phase 2
Phase 3
Phase 4
Phase 5
You should get to level 3 or above after harvesting.
Then plant marigolds(level 2) or Onion seeds if you got to level 5.
Marigolds are flowers so you plant them in this patch
Do the same thing as you did with potatoes and set the timer for 30 Minutes.
Keep doing this until you are level 9 then buy some Guam Seeds. Now this is what you are mainly going to be doing until 99 farming+. Farm runs.

2. Farming Runs

Spoiler

How to do a farm run.
First start with catherby
Then use a tab to get to ardougne
Go to ::trade and run to that farming patch
Then finish it off with a monsters tele to cave.
Set your timer for 30 minutes again.
Have this inventory.
Any seed, Seed dibber, Secateurs, rake, and an Ardougne Teleport

First get to catherby and harvest or plant a seed/herb. Steps to get to Catherby are in the 1st section.

Now buy a "Ardougne teleport" and "Falador teleport" from Herquin at ::shops
Follow the path to your herb patch and harvest or plant a seed/herb.
Now that you're done with Ardougne you're going to need to go to the falador herb patch!

To get the falador herb patch you can simply use your falador tab and run to the patch
Now you need to do the last part of the farm run.
Use the monsters teleport on your spellbook and go to cave

Your herb can become diseased if you dont use compost but it's rare enough to not use.(I would say that at least.) If your herb does become diseased you can easily buy a plant cure from the farming shop in catherby herb patch. Just use the plant cure on the diseased herb and it will continue to grow.

Your herb can also die if it has been grown to long. When your herb is dead it is purple and you need to use a spade on the patch to plant a new seed.

Some tips are to have your supplies ready to go. You should have your four farming tools, seeds, and Ardougne teleport tab close by to eachother.

3. Xp Rates

Spoiler

Here are the following xp rates. You should use the best herb for yours farm runs that you can use to get the most xp.

4. How to get seeds

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There are 4 ways to obtain seeds.

The first way to obtain seeds is by killing chaos druids. These druids are very weak and a level 3 could kill them. Any weapon and armor is appropriate here. To get to chaos druids you need to go to your spellbook and use monster teles and then click cave. They do not drop tortsol seeds sadly.

The second way to obtain seeds is by pickpocketing Master Farmers. You need 38 thieving to pickpocket him. He's located at the catherby herb patch. He gives all kinds of seeds.

The forth way of getting seeds is by slayer tasks. THIS IS THE ONLY WAY TO GET TORTSOL SEEDS(Besides trading). On the last kill of your slayer task there is a chance that the monster will drop seeds. These seeds could be tortsol or others.

5. Money Making with Farming

Spoiler

A good way to make money with farming is by farming toadflax's. They go for 1m each and are in high demand since they are the best herblore xp, the only downside is that they don't give much xp. A few other herbs that are popular are ranarr's since they give the 2nd best xp.
Farming toadflax's can get you up to 32m-64m per hour, But you could have multiple accounts farming them to increase profit.

6. Prestiging

Spoiler

Once you achieve 50m farming xp you can prestige. The prestige item gives you double the farming experience while wearing it. You can talk to bob at ::shops to prestige. The prestige item is magical secateurs
If you ever make it to 200m farming you can prestige it for a 2nd time and receive an untrimmed farming cape. This give triple the farming xp while wearing it.